Change ctrl s to download .scn file instead of json

[lalalune]

Describe your changes

Saves a .scn when user presses ctrl s, which can be easily re-uploaded. Requested by creators to speed up development workflow and use the app as an editor.

What are the steps for a QA tester to test this pull request?

Press ctrl s Scene should be saved Verify that scene works by dragging back into window to load

Issue ticket number and link

https://github.com/webaverse/app/issues/3652

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [x] I am not adding any irrelevant code or assets
• [x] I am only including the changes needed to implement the change
• [x] I have playtested and intentionally tried to find error cases but couldn't

[lalalune]

Closed in favor of Upstreet PR